In current data communication, an access network becomes infrastructure of a communication network since the access network is a transport entity of a Service Node Interface (SNI) and a related User Network Interface (UNI). Because of the feature, the access network should adopt a fair, flexible and secure multi-address technology. A PON, due to features of wide frequency band, large capacity, convenient capacity expansion and applicability to high-speed data transmission, becomes a hot technology of an optical access network and further is the most popular technology in the current optical access network technologies.
A typical PON, as shown in FIG. 1, includes an OLT, an ODN and an ONU, and may further include an Optical Network Terminal (ONT), wherein one or more ONTs are connected with the ONU and serves as specific user(s) of the ONU.
Respective main part of the typical PON and function thereof are as follows:
the OLT, which mainly provides an optical interface between a network and an ODN, and can separate exchange services from non-exchange services, manage signaling and monitor information from an ONU and provide maintenance and supply capability for the OLT per se and the ONU;
the ODN, which is connected with the OLT and the ONU mainly through one or more optical splitters, takes charge of distributing downlink data and integrating uplink data, finishes power distribution, wavelength multiplexing and the like of optical signals, and generally adopts a tree branch structure; and
the ONU, which provides user data, video, and an interface between a telecommunication network and an optical network, converts received optical signals into signals required by a user, and cooperates with the ONT to form a network terminal.
At present, there are three types of PONs based on different multiplexing technologies, i.e., a PON based on Time Division Multiplexing (TDM-PON), a PON based on a Wavelength Division Multiplexing (WDM-PON) and a PON based on Optical Code Division Multiple Access (OCDMA-PON).
The TDM-PON is the most mature PON technology; an Ethernet Passive Optical Network (EPON) and a Gigabit Passive Optical Network (GPON) widely used at present both belong to the TDM-PON technology. Both uplink and downlink of the TDM-PON system use the TDM technology, each utilizing a wavelength. Although the TDM-PON has advantages of mature technology, lower cost and the like, it is very difficult to realize an electricity based high-speed burst receiving technology when a higher bandwidth is expanded; it is not only needed to add a complex bandwidth management algorithm, but also proposed to have a demanding requirement on semiconductor and optoelectronics industries in the aspects of clock synchronization and fast optical signal detection. In addition, the TDM-PON technology further has problems of fragile network system security and difficult fiber fault location.
With continuous increases of bandwidth requirements and user number, a Wavelength Division Multiplexing (WDM) technology is gradually introduced to an access network and is combined with a PON to form a WDM-PON network. Multiple users share the same optical fiber in the WDM-PON network, however, different users are allocated with different wavelengths; in this way, a bandwidth utility ratio can be provided. The WDM is divided into Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM), wherein a channel spacing of the CWDM is 20 nm while a channel spacing of the DWDM is 0.2 nm to 1.2 nm. A G1983 standard having formulated by an International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is only applicable to the WDM technology of 113 μm/115 μm, i.e., the CWDM. The number of wavelengths that the DWDM can provide is greatly increased. However, compared with other wideband access, an initial investment of the WDM-PON is huge. Furthermore, various photoelectric devices required by the WDM-PON are immature, for example, a multi-frequency laser, a broad-tuning single frequency laser, a Light-Emitting Diode (LED) of an integrated amplifier and the like have not entered a large-scale commercialized application, which is a key of the WDM-PON moving towards marketization.
The OCDMA is a multi-address multiplexing technology which combines a large bandwidth of an optical fiber media and flexibility of Code Division Multiple Access (CDMA). The OCDMA is highly concerned in respect of upgrading a present PON system or serving as a main technology of a next-generation PON, since the OLT and ONU the OCDMA system can use a relatively simple multiplexing/multiple access function without needing synchronization, the present PON does not have to make great upgrade for being applied to the OCDMA; in addition, some attractive technologies of the OCDMA per se, such as all-optical processing, real asynchronous transmission, soft capacity, transparent agreement, flexible control of Quality of Service (QoS) and the like, make a research on the OCDMA-PON be increasingly concerned by people.
However, the OCDMA-PON system has inherent defects as follows: the number of code multiplexing is limited, which then limits the number of access users of the system; as an increase of multiplexing number, a crosstalk between users is increased gradually, which influences the number of access users of the system to some extent; and a greater bandwidth needed by the OCDMA system which adopts a spread spectrum technology and a Bit Error Rate (BER) inherent defect caused by interference between users influence the number of access users of the system.
An Optical encoder/decoder is a core part of the OCDMA system. At a transmitting end, the optical encoder converts data bits into a spread spectrum sequence; and at a receiving end, the optical decoder restores the spread spectrum sequence to the data bits by using a related decoding principle. In a CDMA communication system, all users occupy the same frequency band and time of the same channel together, and signals that different users use to transmit information are distinguished by different encoding sequences, that is, each user is allocated with a pseudorandom sequence. At a transmitting end, information of each user generates a pseudorandom sequence through the optical encoder/decoder; since the encoder/decoder is unique, the pseudorandom sequence is unique too; each information bit of a user is encoded into a pulse string; and at a receiving end, a user uses the encoder/decoder corresponding to the same pseudorandom sequence to perform a related operation, ao as to restore the transmitted information. The pseudorandom sequence is called an address code of an user, and each encoding pulse is called a chip. The optical encoder/decoder plays a role of encryption and decryption on optical signals, thereby increasing security of a network. At present, main types of the OCDMA encoder/decoder includes: a time domain encoding/decoding scheme based on optical delay line, a frequency domain encoding/decoding scheme based on diffraction grating and phase mask, an encoding/decoding scheme based on Fiber Bragg Grating (FBG) and an encoding/decoding scheme based on Array Waveguide Grate (AWG).
A hybrid PON is a PON adopting the above two or more technologies.